The present invention relates to a thin-film electroluminescent (referred to as "EL" hereinbelow) display panel and, more particularly, to a method for aging such a thin-film EL display panel.
Firstly, a conventional electroluminescent (EL) display panel is illustrated in FIG. 1, wherein the EL display panel comprises a first transparent glass substrate 1, a transparent electrode 2 made of In.sub.2 O.sub.3, SnO.sub.2 etc. formed thereon, a first dielectric layer 3 made of Y.sub.2 O.sub.3, TiO.sub.2, Si.sub.3 N.sub.4, SiO.sub.2, etc., an EL thin film 4 made of ZnS:Mn, and a second dielectric layer 5 made of a material similar to the first dielectric layer 3. A counter electrode 6 is made of Al and is formed on the second dielectric layer 5 through evaporation techniques. The first dielectric layer 3 is provided by sputtering or electron beam evaporation techniques. The EL thin film 4 is made of a ZnS thin film doped with manganese at a desired amount. An AC electric field from an AC power source 7 is applied to the transparent electrode 2 and the counter electrode 6 to activate the EL thin film 4.
The EL thin film 4 is fabricated by electron beam evaporating a ZnS sintered pellet doped with Mn at a preferable quantity and, then, by heat-treating it in vacuum or an inert gas atmosphere. Mn serves as a luminescent center in the EL thin film 4.
It is necessary so conduct an aging procedure, with a voltage applied, on the thin-film EL display panel so as to stabilize its optical properties such as brightness of emitted light, and its physical properties such as dielectric breakdown properties.
In order to shorten a time required to complete the aging procedure, it was generally presumed that the magnitude, pulse width, and/or frequency etc., of the voltage applied to electrode means of the thin-film EL display panel became higher.
However, if this was actually conducted, a disadvantage was the generation of remarkable dielectric breakdown.
In view of the foregoing, it was usual that the aging procedure was conducted with a constant AC voltage lower than a voltage applied to provide electroluminescence. As a result, a long time of about 50-60 hours was conventionally required to complete the aging procedure.
Further, according to the conventional aging procedure, properties for representing the relation between emission brightness of the electroluminescence and the voltage applied to the thin-film EL display panel (which is referred to as "B-V" properties hereinafter) vary during the aging procedure. This varies the aging efficiency with the lapse of time while a constant voltage is applied across the transparent electrode 2 and the counter electrode 6 for aging purposes.
FIG. 2 shows a graph of the B-V properties of the thin-film EL display panel which is subjected to the conventional aging procedure in which a constant voltage is applied across the electrodes. In the graph of FIG. 2, the respective data are related to the following conditions:
l.sub.1 : before the aging procedure
l.sub.2 : at about 1 hour after the aging procedure
l.sub.3 : at abut 3 hours after the aging procedure
l.sub.4 : at about 5 hours after the aging procedure
This indicates that an operation point on the B-V properties varies with the lapse of time. However, to efficiently perform the aging procedure, it is preferable that some operation points corresponding to voltages properties are constant as much as possible.
FIG. 3 shows a graph representing changes in electroluminescence brightness with the lapse of time during the aging procedure when the conventional aging procedure using a constant voltage is performed.
FIG. 4 shows a graph representing changes in a voltage V.sub.th for starting to emit the electroluminescence of 1 foot-lambert (ft-L) with the lapse of time during the conventional aging procedure, from its initial value before the aging procedure. Normally, the voltage V.sub.th is in the order of about 150 V to about 190 V.
In order to shorten the aging time for sufficient results, the present inventors have offered the conception that an appropriate AC voltage pulse, for example, more than (V.sub.th +30 volt) and less than (V.sub.th +60 volt) is used for aging purposes.
However, according to the conventional aging procedure using the constant voltage, the B-V properties and the value of V.sub.th are changed as indicated in FIGS. 3 and 4, so that an electroluminescence intensity and the voltages are made insufficient. Even when an initial setting voltage is made higher to compensate for reductions of the intensity and the voltage, remarkable dielectric breakdown is generated in many thin-film EL display elements. Consequently, it was highly desired to shorten the aging time and compensate for the changes in the B-V properties and the value of V.sub.th of the thin-film EL display element.